1. Field of the Invention
The invention disclosed in the present specification relates to a liquid crystal panel which can display images favorably and to equipments using such liquid crystal panel.
2. Description of Related Art
There has been known an active matrix type liquid crystal display having a structure in which an active matrix circuit, a circuit for driving the active matrix circuit (referred to as a peripheral driving circuit) and other peripheral circuits (various circuits required for a liquid crystal panel) are integrated on one and the same substrate by TFTs. This structure is called a peripheral driving circuit integrated type liquid crystal display.
This structure is characterized in that:
(1) the whole construction may be simplified and miniaturized;
(2) its fabrication process may be simplified; and
(3) it is advantageous in lowering power consumption.
Because it is required to reduce the size of the liquid crystal panel used in portable information processing terminals, a projector type liquid crystal display and the like, it is very useful to integrate the peripheral driving circuit to that end.
According to the finding of the inventors et. al., the existence of spacers which decide a thickness of a liquid crystal layer causes a problem in miniaturizing the liquid crystal panel.
For instance, because the size of a pixel is around 30 to 40 μm square or less in a small liquid crystal panel which is used for a projector and the like and whose size across corners is less than 2 inches, its display is influenced by the shadow of the spacers which exist within the unit pixel and whose diameter is several μm.
In order to solve such a problem, it is conceivable to arrange so as to maintain a cell gap (which is defined as corresponding to the thickness of the liquid crystal layer) by a sealing member (sealant) disposed so as to surround a pixel region (pixel matrix region), without using the spacers.
FIG. 2A schematically shows a transmission type liquid crystal panel which has been made in trial to observe a cell gap and whose screen size across corners is 1.4 inches. Because this liquid crystal panel is a prototype, no active matrix circuit nor peripheral driving circuit are formed.
In FIG. 2A, one denoted by the reference numeral 21 is a sealing member and liquid crystal is filled on the inside thereof. This liquid crystal panel is arranged such that the cell gap is maintained by a material called filler contained within the sealing member 21 and having a predetermined size.
No spacer for maintaining the cell gap is used in this liquid crystal panel. That is, no means for maintaining the cell gap is provided except of the region where the sealing member is provided.
A liquid crystal injection port 24 is closed by ultraviolet hardening resin or the like after injecting the liquid crystal. The reference numeral 23 denotes a glass substrate composing the liquid crystal panel. The figure shows a state in which two glass substrates overlap each other (the liquid crystal is maintained between these two glass substrates).
Because no spacer exists in the pixel region (inside of the sealing member 21 in this case) in this arrangement, no spacer affects the image quality.
However, when monochromic light is irradiated to the liquid crystal panel shown in FIG. 2A, concentric interference fringes 25 are observed from the region where the sealing member is provided to the center of the liquid crystal panel as shown in FIG. 2B.
The interference fringes 25 indicate that the interval of the cell gap is not uniform. That is, it indicates that the cell gap is narrow at the center part of the liquid crystal panel.
FIG. 3 is a schematic diagram wherein this state is stressed extremely. FIG. 3 shows a state in which liquid crystal 22 is interposed and maintained between a pair of substrates 201 and 202 by the sealing member 21. It also shows that a cell gap d is small at the center part 203 of the liquid crystal panel.
The interference fringes shown in FIG. 2B appear at part 204 where the positional variation of the cell gap is large. No interference fringe appears at the center part 203 of the liquid crystal cell because the cell gap barely varies there.
The interference fringes as shown in FIG. 2B is caused by interference of light reflected from the surface of the substrates composing the liquid crystal cell.
The principle of causing the interference fringes will be explained with reference to FIG. 4. FIG. 4 shows a state in which a pair of glass substrates 41 and 42 face each other while keeping the gap (corresponds to the cell gap) thereof inconstant.
When monochromic light 43 is input in such a state, light reflected by the back face of one glass substrate 41 interferes with light reflected on the surface of the other glass substrate 42 (although there exists other interferences, it is considered as such in order to simplify the discussion).
If the cell gap d is constant, i.e. if d1=d2=d3, no bright and dark stripe pattern appears because the state of the interference is the same at any part.
However, when the gap d is different depending on locations, the bright and dark conditions differ depending on the locations, causing a stripe pattern in accordance to the state of changes of the gap d.
The stripe pattern is assumed to be caused when there is a difference of about λ/2 in the difference of the cell gap at each location.
When a cell gap at a first bright part is d1 and a cell gap at a neighboring second bright part is d2, a difference of an optical path length at those two locations is 2(d2−d1) considering that the light reciprocates in the cell gap.
The first and second bright conditions hold when there is a difference of about a wavelength λ of the incident light 43 between the two optical path lengths.
Accordingly, an expression λ=2(d2−d1) holds. That is, the difference (d2−d1) of the cell gap of the two bright parts is about λ/2.
Because the wavelength λ of the incident light 43 is around 500 to 550 nm (0.5 to 0.55 μm) in general, the difference of the cell gaps is estimated to be around 0.25 μm.
Actually, because the state of reflection of light on the surface of glass is not so simple (it is complicated when an insulating film, a conductive film or the like is formed on the surface of the glass substrate) and wavelength of light within a liquid crystal material turns out to be λ/n by refractive index n of the liquid crystal material when the liquid crystal material is filled therebetween, the above-mentioned difference of the cell gaps will be smaller than the estimated value.
In any case, the stripe pattern is observed when the cell gap varies in the order of about several tenth of visual light.
Generally, the cell gap of the liquid crystal panel is set around at 1 to 5 μm. This value is decided depending on operation modes or a liquid crystal material to be used.
However, when the cell gap of the liquid crystal panel deviates by 10% or more from the preset value, transmittancy of light transmitting through the liquid crystal changes by around 20% or more of the preset value.
Accordingly, when the liquid crystal panel wherein the interference fringes as shown in FIG. 2B are observed is fabricated by setting the cell gap at 3 μm and the whole of the inside of the sealing member is used as a display screen, a difference of cell gap in the order of μm is produced between the peripheral part and the center part of the screen.
This means that the state of display might differ remarkably at the peripheral part and the center part of the screen (in fact, a screen display whose image quality differs in the shape of donuts is observed).
Accordingly, it is an object of the invention disclosed in the present specification to solve the above-mentioned problem of non-uniformity of the cell gap in the liquid crystal panel.